The present invention relates to electrodes for use in magnesium production by electrolysis of molten salts.
In electrolytical production of magnesium it is extremely important to obtain good wetting of the cathodes with the magnesium. Inferior wetting conditions result in fine droplets of metallic magnesium which easily recombine by recirculation and prolonged stay in the anode compartment. This risk decreases with increasing diameter of the metal droplets. Normally cathodes of steel are employed due to the materials stability, but unfortunately the material has poor wetting properties. By using steel cathodes the produced magnesium will be contaminated by iron due to corrosion of the cathodes. A typical content of iron in the produced metal will be in the order of 3-400 ppm.
Use of bipolar electrodes for magnesium production is also of current interest. Electrodes of this type are for example described in EPO patent No. 0,0027,016. In common use for this type of electrode, plates of iron and graphite are joined together face to face where the graphite is acting as anode and the iron acting as cathode. As with monopolar electrodes the produced magnesium will have a small content of iron. For this type of electrode it is also extremely important to have good wetting conditions between the produced metal and the cathode.
U.S. Pat. No. 4,073,704 describes a process for the production of metallic magnesium wherein a molten salt bath containing sodium chloride, magnesium chloride, potassium chloride, calcium chloride and magnesium fluoride is electrolytically decomposed. The cathodes are made of steel and the anodes of graphite. To the electrolyte there is periodically added inorganic salts of molybdenum or tungsten, metallic molybdenum or tungsten, or mixtures thereof. According to the patent this will give a coating of molybdenum or tungsten on the cathode surface. This coating is said to be less than 15 angstroms and will promote the wetting of the cathode with magnesium. Less sludge will be produced, and the sludge will also contain less magnesium.
By using the process according to U.S. Pat. No. 4,073,704 molybdenum or tungsten periodically must be added to maintain the coating, and it is difficult to control the deposition of the coating metals on the cathode. Both molybdenum and tungsten have high density, and by adding metallic Mo and W they might be precipitated in the electrolytic bath without being chlorinated and deposited at the cathode. Another disadvantage is that molybdenum and tungsten can be deposited upon already liberated magnesium, and thereby it will be removed together with the produced metal. This will contaminate the metal. By adding metal oxides of molybdenum and tungsten there will be a high content of oxides in the bath which can cause oxidation of the cathode coating and thereby make it inactive. Magnesium can also be deposited as an oxide. The cathodes thereby can be coated with an oxide if there is too much contamination in the electrolytic bath. One condition for the functioning of a process like this, is that there are few contaminants present in the electrolytic bath.
By such a process the cathodes will be unevenly and randomly coated with molybdenum and/or tungsten, and some parts of the steel surface will also be without coating from time to time.
This can lead to some corrosion of the iron which will contaminate the produced magnesium.